This invention relates to electronic measurement circuits used primarily for measurement or detection of audio signal level and more particularly to measurement circuits designed to produce an output proportional to the logarithm of the root mean square or RMS value of an input signal. RMS conversion is commonly known and used for the measurment of audio signal levels and many systems exist which utilize the process.
Some systems apply the input signal to a full wave rectifier circuit and then process the rectified output by a unipolar log converting circuit consisting of an inverting stage operational amplifier with a single feedback path through two semiconductor junctions. Such systems place stringent requirements on the low level accuracy of the full wave rectifier circuit. The inaccuracies become readily apparent at signal levels below 20 MV.
This problem is sometimes overcome by processing the input signal through a bi-polar logarithmic converter which provides greater accuracy at extremely low signal levels. However, such systems require complex external circuitry in order to control the output ratio of volts-per decibels of input signal and to adjust the output dc level at a predetermined voltage. Since easy external control of threshold and ratio functions is a desirable feature of many circuit designs using RMS detection, this solution is often inadequate.